Both flashing would save more battery than 1 solid and 1 flashing. Your solid one will go dim or off sooner.
Maybe yes, maybe no.
The Cateye Reflex flasher is good for 120 hours on the setting I use. My solid OE fender light is dyno powered, therefore has unlimited run time. I'm sure with a little research one could find flasher, and solid lights with similar run time if that's a concern.
While I have observed that a solid and flasher used together offer the "best" perceptive visibility, that in no way means two flashers are "bad", especially if using 2 flashers with dissimilar patterns.
It really is splitting hairs. The only really bad options are little or no lights, or lights that overwhelm those viewing them.